1. Field of the Invention
This invention relates to a video signal encoding method, a video signal encoding apparatus, a video signal transmitting method, and a recording medium in which the encoded data generated by the video signal encoding method or apparatus have been recorded, and more particularly is applicable to the case where predetermined image data are divided into a plurality of hierarchical image data respectively having different resolutions and the plurality of hierarchical image data are encoded respectively to generate encoded data (i.e., image data is hierarchically encoded).
2. Description of the Related Art
Conventionally, in a video signal encoding apparatus of the type mentioned above, high resolution input image data is defined as first hierarchical image data, and second hierarchical data having a resolution lower than that of the first hierarchical data, third hierarchical data having a resolution lower than that of the second hierarchical data, and so on are sequentially formed from the first hierarchical image data. Then, the plurality of hierarchical data are compressively coded, respectively. Therefore, hierarchically encoded data are formed for a plurality of levels having gradually reduced amounts of information. The plurality of hierarchically encoded data are transmitted through a communication path or a recording/reproducing path.
A video signal decoding apparatus which decodes the plurality of hierarchically encoded data is capable of entirely decoding the plurality of hierarchically encoded data as well as selectively decoding a desired one of the hierarchically encoded data in accordance with the resolution or the like of a television monitor corresponding thereto. In this way, when desired hierarchical data only is decoded from a plurality of hierarchical data, desired image data may be produced with a minimally required amount of transmitted data.
Referring now to FIG. 1, a video signal encoding apparatus 1 for realizing, for example, four levels of encoding as the hierarchical encoding comprises thinning filters 2, 3, 4 and interpolation filters 5, 6, 7 at three stages, wherein input image data D1 is processed by the thinning filters 2, 3, 4 at the respective stages to form reduced image data D2, D3, D4 having sequentially lower resolutions, and the interpolation filters 5, 6, 7 restore the reduced image data D2, D3, D4 to have their original resolutions before the reduction.
The outputs D2 to D4 of the respective thinning filters 2 to 4 and outputs D5 to D7 of the respective interpolation filters 5 to 7 are inputted to differential filters 8, 9, 10, respectively, to generate differential data D8, D9, D10. Since the frequency distribution of the differential data D8 to D10 concentrate in the vicinity of zero, the video signal encoding apparatus 1 can reduce the amounts of respective hierarchical data as well as the signal power thereof. Further, the runlength coding or Hoffman coding, etc. is used so as to reduce the amounts of data. Here, the differential data D8 to D10 and the reduced image data D4 are in a dimensional relationship in which the differential data D9, D10 and the reduced image data D4 respectively have sizes equal to, a quarter of, and a sixteenth of the size of the input image data D1.
The differential data D8 to D10 generated by the differential circuits 8 to 10 and the reduced image data D4 generated by the thinning filters 2 to 4 are encoded or compressed by encoders 11, 12, 13, 14, respectively. As a result, first, second, third, and fourth hierarchical data D11, D12, D13, D14 having different resolutions from each other are sent from the respective encoders 11, 12, 13, 14 to a communication path in a predetermined order or are recorded in a recording medium through the transmission path.
The first to fourth hierarchical data D11 to D14 thus transmitted are decoded by a video signal decoding apparatus 20 illustrated in FIG. 2. Specifically, the first to fourth hierarchical data D11 to D14, which are supplied from the communication path or the recording medium via an input terminal, are decoded by decoders 21, 22, 23, 24, respectively, and as a result, the decoded fourth hierarchical data D24 is outputted from the decoder 24.
An output of the decoder 23 is added to interpolation data for the fourth hierarchical data D24 generated by an interpolation filter 26 in an adder circuit 29 to restore third hierarchical data D23. Similarly, an output of the decoder 22 is added to interpolation data for third hierarchical data D23 generated by an interpolation filter 27 in an adder circuit 30 to restore second hierarchical data D22. Further, an output of the decoder 21 is added to interpolation data for second hierarchical data D22 generated by an interpolation filter 28 in an adder circuit 31 to restore first hierarchical data D21.
However, in the video signal encoding apparatus 1 for realizing the hierarchical encoding as mentioned above, the input image data D1 is divided into a plurality of hierarchical data for the encoding, so that the amount of data to be transmitted is necessarily increased by the number of hierarchical levels. Thus, the video signal encoding apparatus 1 has a problem that its compressing efficiency is correspondingly reduced as compared with a highly efficient encoding scheme which does not employ the hierarchical encoding.
In the video signal decoding apparatus 20, in turn, the hierarchical data D24, D23, D22, D21 are orderly restored from the highest hierarchical data toward the lowest hierarchical data. Stated another way, lower level hierarchical data is restored based on restored higher level hierarchical data. Thus, the video signal decoding apparatus 20 has a problem that when the encoded higher level hierarchical data such as D14 and D13 are decoded, if errors due to the compressive coding are included in the decoded data, the compressive coding errors are propagated to the decoded lower level hierarchical data D21, D22 for which a high resolution is essentially desired, and a deteriorated image quality due to the compressive coding errors prominently appears in the restored lower level hierarchical data.